Wireless charging or powering of devices in general is an established technique that is convenient to users. Wireless powering can also be used in harsh environments where corrosion or moisture might jeopardize functionality or safety when galvanic contacts are used. There are several standards for wireless power such as Qi, PMA, Rezense and WiPower, and the market is growing rapidly. These techniques are mostly used for charging a battery powered device (e.g. a mobile phone, a tablet computer, etc.). Charging of multiple devices is possible. For instance in the Qi standard power plates with many smaller coils are available, however the devices need to be precisely positioned adjacent to each other (in the horizontal plane).
High-end patient monitoring is expanding from its traditional application in the critical care arena (ICU, OR) towards lower acuity settings such as the general ward, hospital-to-home, connected primary care, etc. The success of the existing high-end products is due to the quality of the measurements, their modularity, the overall system connectivity, the user interface and its consistency (backwards compatibility) across the total product line. At the same time the value segment market is expanding rapidly to address emerging countries and lower acuity settings where low-cost is of prime concern. In these markets compromises may be made on modularity, connectivity and (sometimes) measurement quality.
In the lifestyle and sports arena also physiological measurements are used more and more (such as heart rate, respiration rate, SpO2).
In said new application spaces wearable (cordless) sensors, miniaturization and low-power are necessary. The basic requirements across all these segments are the same, namely excellent measurement quality compared with non-compromised electrical patient safety. The latter is strictly regulated in the IEC 60601 standard and dictates in a worst case scenario (direct connection to the heart) a 10 μA maximum leakage current, 4 kV isolation towards ground and 1.5 kV isolation between each of the measurements. Additionally, the patient monitor must be able to withstand high differential voltages introduced by a defibrillator and large RF voltages from a surgical knife.
Conventional isolation and protection concepts are based on inductive power couplers (transformers) and optical data couplers for data transport, next to maintaining sufficient creeping and clearance between PCBs and connector pins.
Synchronization of clocks in computer networks and sensor systems is a well-known problem and solutions are available for both centralized and distributed systems. Synchronization of vital sign waveforms in patient monitoring is a major challenge, where requirements are severe. It is important that for instance waves shown on the display correspond beat by beat. Moreover the delay from signal to signal (for instance ECG to Invasive Blood Pressure (IBP) is equally important as it may contain important clinical information. Recently continuous non-invasive blood pressure measurements have been proposed based on Pulse Arrival Time (PAT) or Pulse Transit Time (PTT). For these methods timing errors between sensors must be smaller than 1 ms. Generally speaking, monitoring applications require sub millisecond timing accuracy.
Present solutions often use one master clock and multiple slave clocks in the devices connected via a cabled network. Messages are sent via the network to synchronize clocks. It is not straightforward to apply such techniques in a wireless sensor system as is used in wearable patient monitoring devices. Sometimes a master clock signal is transmitted but timing accuracy is typically a few ms to 100's of ms depending on the radio standard. Although much better timing sync is in principle possible for a radio (like Wi-Fi or Bluetooth (BT) micro second time stamps synchronization) such accuracy can only be reached with dedicated low-level implementation, not on top of a regular radio protocol implementation. Furthermore, the modules generally each have a separate module clock with its own drift. This requires calibration at regular intervals.
Other methods rely on the emission of a common time stamp signal and then derive timing signals locally in the devices based on their own local crystal oscillator. However, many standard radio protocol implementations do not allow synchronization at sub-millisecond accuracy, and in practical wearable applications the radio link may not always be available, while the medical application still requires continuous local signal synchronization.
Hence, there is a need for a solution that meets stringent relative time errors, for instance for a sensor system comprising dielectrically isolated nodes with only one power source. Moreover, there is a need for a solution that requires only minimal increase in complexity to achieve the required specifications (i.e. drifts less than 1 ms) over a period of several hours or days.
US 2007/0254726 A1 discloses an apparatus including a wireless transmitter modulating a carrier wave by transmission data and wirelessly communicating a signal, a wireless receiver mixing the wireless transmitter signal and a carrier wave and receiving the transmission data, a power carrier wave clock generator provided on one of the wireless transmitter and receiver generating a power carrier wave clock, a non-contact power transmitter transmitting power between the wireless transmitter and receiver through electromagnetic induction from the power carrier wave clock, a carrier wave generator mounted on the one of the wireless transmitter or receiver, and generating a carrier wave based on the power carrier wave clock, and a carrier wave reproducer mounted on the other of the wireless transmitter or receiver, and reproducing a carrier wave having the same frequency as the carrier wave based on a clock having the same frequency as the power carrier wave clock.